You also need to be aware of the different roles each filter has. An OIII will show up planetary nebulae, but not an UHC type. UHC filters are best for emission nebulae, an OIII will do so too, but not as dramatic. Very different applications.
This following link to the Lumicon filter guide is a great tool for familiarizing oneself with the many different filters. It will help you understand the differences between OIII, UHC & Hbeta:
http://www.lumicon.com/store/pg/15-L...a-Filters.aspx